A replica LeMat revolver I dressed up with some copper tubing, a pressure regulator thingy, and some holes.
When using this, I crammed some glowsticks down the lower barrel so that cool arcane light would glow from the holes. This didn't work very well, so I'm thinking of coming up with some sort of LED lighting to put in there.
This replica is a non firing prop manufactured by Denix.
A real LeMat is a French made percussion cap revolver that holds 9 .41 caliber rounds in its cylinder, and a shotgun round in the large underbarrel. The shotgun round is fired by flipping the striker on the hammer down to hit the lower firing pin. Pretty nifty for Civil War era technology.